Since its launch in 2014, Facebook's ad network has limited itself to mobile app placements, choosing to avoid working with mobile web properties. Now that's changing.

AdExchanger has learned Facebook Audience Network is one month into a test involving about 10 publishers that would see the ad network's placements run on mobile web pages. The expansion brings its own set of technical hurdles, along with a large revenue expansion opportunity for Audience Network, which reached a $1 billion run rate last quarter.

BuzzFeed-like social publisher Diply is among the publishers in the closed beta test. A Facebook rep confirmed the test and Diply's involvement, but declined further comment.

“This is Facebook coming in and offering an alternative to AdSense,” said a source with knowledge of the test who did not want to be identified revealing private information.

The source predicted that Facebook’s foray into mobile web inventory could chip away at Google’s dominance as a broker of publishers’ ad inventory.

Facebook's foray into mobile web inventory will expose Audience Network to a new group of publisher partners. Mobile web is where both big comScore 500 publishers and long-tail media companies play. The expansion could give Google, Apple, AOL-owned Millennial Media and other mobile ad network operators a run for their money as Facebook seeks to become a reliable, must-include source of revenue for publishers.

While it is early to discuss results, the source familiar with the test data said Facebook Audience Network has fairly high fill rates. The ads have been served through a publisher’s server, quite often DoubleClick for Publishers.

Facebook first rolled out Audience Network nearly two years ago, and the acquisition of LiveRail later that year paved the way for it to add native video to FAN, which it did last August. That business now generates a significant amount of revenue.

According to Facebook, its audience network generated a $1 billion annual run rate using (high) Q4 2015 ad spending as a benchmark. Most of that money went to publishers, and Facebook did not disclose its net revenue after media suppliers were paid.

That revenue was generated with a fairly low market share: Apps that use FAN account for 6% of time spent on mobile, according to Facebook’s data.

Like Google AdSense, Facebook has unique demand to offer. Publishers get access to social media budgets and local advertisers are also in the mix. Facebook boasts access to more than 2 million paid advertisers, many of whom use its Power Editor tools to manage their campaigns. Google reached 1 million advertisers in 2007, the last time it disclosed its advertiser count.

By starting its audience product in mobile apps, then adding video and now mobile web to Audience Network, Facebook has left some in the market wondering whether it will eventually add desktop media, the most mature and saturated market and one where AdSense is well-established.